“If, in fact, and it’s a subjunctive, Trump knew about the Russian efforts on his behalf, then there is a live question about whether he has been giving aid and comfort to the enemy, which is the definition of treason in the Constitution,” Meacham, who has written biographies on past presidents Thomas Jefferson and George H.W. Bush, said on MSNBC's "Morning Joe."

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“So, this isn’t just a who-wins-the-week, who-loses-the-week conversation," he continued. "This is an existential constitutional crisis, because it’s quite possible that the president of the United States right now is a witting or at least partially witting agent of a foreign power, and I say that with great care, but that’s a possibility.

"And I think that we have to somewhat tap the brakes on our ordinary assessment of things and realize that this is possibly the biggest kind of question we could confront as a democracy."

Meacham's remarks came as he discussed the Watergate investigation of President Nixon. He noted that Nixon's downfall coincided with investigations and an economic downturn.

"What begins to happen in 1974, is you have the investigations, but you also have the economy souring," he said. "And that’s something to watch. One barometer about what’s going to happen to President Trump is ... what the markets are doing."